This invention is particularly concerned with thermoforming operations in which a web or sheet of thermoplastic material is first softened by heat and then formed or shaped while in this stretchable plastic state by being drawn or pressed against the contours of a mold or die under the influence of a fluid pressure differential. The former may be exemplified by the application of pressure within a pressure box produced by compressed air or the like, to press the softened material against the contours of a male mold or core pin. Alternately, a vacuum may be applied from within a female mold while allowing atmospheric pressure to shape the heat softened thermoplastic material against the surface of the female mold. For many purposes, a combination of these two techniques is preferred with the softened sheet material being subjected to vacuum on the face contacting the mold surfaces and to a substantial positive pressure on its other face as such combination provides greater versatility and better control in the forming operation.
Development of the container art has been directed to the use of containers having large capacities, particularly with the ever increasing cost of materials of construction. Present processes for producing tall containers having large length to diameter ratios have almost exclusively utilized extrusion blow molding techniques. In U.S. Pat. No. Re. 28,062 to Scalora, there is disclosed a process for the injection blow molding of a hollow article wherein the parison is extended, preferably in a blow mold prior to expansion of the extended parison to biaxially orientate the plastic material forming the container.